The technical field of the invention relates to line cards (line card systems) or subscriber line interface circuits, the basic function of which is to transmit the encoded voice signals from a digital PCM interface onto the line (and vice versa). In this case, the line, comprising a tip wire and a ring wire or A wire and B wire, needs to be operated using low-frequency programmable voltages.
In addition, the line card has the function of monitoring the signals on the line and external circuits without using a specially provided test unit. By way of example, it is an aim to measure the unknown resistance between the ring wire and ground (or between the tip wire and ground or between the tip wire and the ring wire) with a predefined level of precision (typically 10%). In the case of known methods for measuring the resistance between the ring wire and ground (the same applies to measurements of the resistance between the ring wire and the tip wire and between the tip wire and ground), the line card needs to be put into a defined measurement mode in which, by way of example, the ring wire has a predefined potential (for example −50 V) applied to it. In a subsequent step, the current flowing between the ring wire and ground is measured by a control unit on the line card. The resistance between the ring wire and ground is calculated from the quotient of the predetermined voltage and the measured current.
A drawback of this known method is that a critical signal-to-noise ratio for the current measurement arises if the resistance which is to be measured is greater than 2 MΩ. At a predetermined potential of −50 V, the current to be measured would then be 25 μA. The high signal-to-noise ratio which arises means that this current level is too low to be able to be measured with sufficient accuracy. From time to time, a tolerance of 10% might no longer be ensured. However, such a tolerance of no more than 10% is demanded by the telecommunication companies in some countries (e.g. Telecom Italia).
The aforementioned problem could be solved by using larger transistors for the subscriber line interface circuit, which are known to have better signal-to-noise ratio characteristics. However, the use of larger transistors would significantly increase the chip area for the line card, which would not be in keeping with a required level of large-scale integration for the components in the subscriber line interface circuit.